Linear encoders are known for providing a signal indication of the position of a movable element along a travel path. In a typical electro-optical incremental linear encoder, an elongated glass or other light transmissive scale is disposed along a travel path and includes an array of regular rulings which provide alternately light transmissive and opaque lines. One or more reticles are provided for movement in relation to the scale, the reticles having a similar array of rulings which are cooperative with the scale array to provide fringe patterns which are sensed to provide an output indication of the position of the reticle in relation to the scale. A light source is provided to illuminate the reticle and scale, the fringe patterns being detected by one or more photosensors. Usually a pair of reticles is employed which are relatively offset to provide photosensor signals which are in phase opposition. A second pair of reticles is often employed to provide a second pair of photosensor signals which are in phase opposition with each other and in phase quadrature with the first pair of signals. These photosensor signals are utilized in associated signal processing circuitry to provide output indications of relative position and sense of movement. Examples of linear encoders are shown in U.S. Pat. Nos. 3,816,003, 3,867,037 and 4,039,826 of the present assignee.
The light source typically includes an incandescent bulb or light emitting diode and a lens for providing a generally collimated light beam for transmission through the reticle and scale patterns. If a single light source is employed to illuminate the several reticles, this source can be quite large in both diameter and depth to accommodate a lens of sufficient aperture to illuminate all of the reticles. In order to reduce the size of the encoder, separate light sources have been employed for each reticle; however, use of such plural light sources can result in variation in the light intensity of the respective light beams, and corresponding variation in the photosensor output signals.